Mountain glaciers around the world, from the Himalayas to the Andes, are shrinking in the face of climate change—and that could pose a major threat to water resources for nearby communities.

Greenland and Antarctica house the world's largest ice sheets, but ice can be found in high-altitude locations around the world, from Asia to Europe to South America. These mountain glaciers are important resources for human settlements. Glacial runoff, especially during the spring and summer, can provide a critical source of fresh water downstream.

But in a new modeling study of 56 glacier drainage basins worldwide, roughly half the studied sites have already reached a kind of tipping point—after which the amount of fresh water that runs off each year begins to decline.

"As glaciers recede, water is released from long-term glacial storage," the researchers note in the paper, which was published yesterday in the journal Nature Climate Change. "Thus, annual glacier runoff volume typically increases until a maximum is reached, often referred to as 'peak water.'" After this point is reached, they note, the amount of annual runoff begins to decline again.

Runoff during the warmest part of the year experiences the sharpest declines, the scientists add, which could threaten the seasonal availability of fresh water. The new study suggests that by the end of the century, about a third of all the drainage basins could experience reductions in runoff greater than 10 percent during at least one month of the warm season.

The research compounds ongoing concerns about water resources in warming mountain environments—not only those fed by glaciers, but also resources dependent on melting snow. Studies suggest that snowpack in mountain regions around the world may be declining annually or melting earlier in the season, allowing for less freshwater runoff later in the summer.

Another recent study, for instance—just out last week in Science Advances — documented a general decline in snow water resources across High Mountain Asia, a region spanning from the Hindu Kush region to the Himalayas.

Now, scientists are increasingly aware that mountain glaciers—like mountain snowpack—are growing more vulnerable to the influence of climate change. Yesterday's study suggests that total glacier volume across all the investigated basins will decrease by about 43 percent by the year 2100, even if the world takes serious steps to mitigate climate change. Under a more severe "business-as-usual" trajectory, in which greenhouse gas emissions continue unabated into the future, these total declines could be as high as 74 percent.

The researchers note that the fate of individual glaciers is still highly variable—some will recede less than others, and water resources could be less affected in some. On the other hand, some sites could experience astonishingly quick or even violent declines.

Another study published yesterday in Nature Geoscience documents the catastrophic collapse of two glaciers in western Tibet. While many factors, including weather and geology, likely played into their rare collapse, the glaciers were also weakened by ongoing climate change, the researchers noted.

The new study reinforces previous research showing the power of climate change over small glaciers worldwide. A groundbreaking 2016 paper, one of the first to conduct a large-scale analysis of shrinking mountain glaciers around the globe, concluded that all but one of the 37 studied sites were "very likely" retreating because of climate change—and at 21 of those sites, the influence of climate change was just about certain. The researchers wrote that the shrinking presented "categorical evidence" of climate change, making mountain glaciers a kind of underrepresented canary in the coal mine for global warming.

Across the board, studies of mountain glaciers increasingly present a worrying reminder that the poles aren't the only parts of the planet suffering major ice loss, scientists say. And while they may not pose the same global threats as those in Greenland and Antarctica, which hold great potential to affect global sea levels in the future, there are still many human communities in their wake that stand to suffer from their loss.

Reprinted from Climatewire with permission from E&E News. E&E provides daily coverage of essential energy and environmental news at www.eenews.net.

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